The manufacture of artificial quarrystone for the construction of mound breakwaters and shore protection structures is a well-known problem. Until now, different solutions have been applied unsuccessfully, through the design of numerous armor units, generally made of concrete, to construct armor layers of mound breakwaters for protection against wind wave attack.
It is worth highlighting that said concrete units for breakwater armor layers are not to be confused with those used for the construction of artificial reefs, since said armor units are designed to resist against large wave breaking and are characterized by specific features such as stability and strength while the artificial reefs are used to improve or protect marine habitats and almost any unit made of any material denser than water is considered suitable.
Concrete armor units for mound breakwaters may be classified according to the design, the construction process and the resistant performance against the wave storms. There are three main methods to place armor units in the construction of an armor layer: (a) random placement, (b) placed in a single layer and (c) regular uniform placement.
The armor units with random placement (a) are the conventional armor units for substituting quarrystone, which are placed in more than one single layer and basically resist by gravity. Both simple and complex armor units can be found within the group of random placement units.
Cubic and parallelepiped blocks are simple armor units, robust, solid, easy to construct and flexible in their performance against the wave storms. Further, they do not undergo significant unit breaking fractures nor do they lose stability, and any errors in design usually increase armor erosion but rarely lead to the complete destruction of the breakwater. However, these are units which have serious drawbacks as regards face-to-face fitting, which can result in significant permeability changes when the packing density increases in the lower areas causing a displacement of armor units from the upper areas (heterogeneous packing). Face-to-face fitting can also result in a reduction in friction with the underlayer and other problems derived from a certain self-arrangement of the armor units which should be placed and maintained in a random position.
The group of complex armor units with random placement includes the so-called “Tetrapod” armor units, which have a higher stability coefficient as a result of their resistance due to interlocking. Nevertheless, the ease with which these units break if they are too large is considered a major drawback. They are brittle and are not resistant to large unit movements; therefore, any error in the design wave storm may lead to the unexpected complete destruction of the breakwater in the first intense storm after construction.
The group of armor units with non-random placement (b) in a single layer includes the so-called “Accropode” and “Core-Loc” armor units which have a good interlocking between units, but have a relatively brittle structural performance because when any of the units fail, a progressive failure takes place. These armor units require less concrete but they must be placed with great care. Any error in the construction or calculation of the design wave storm can lead to the total destruction of the structure.
Finally, the armor units with regular uniform placement (c) may be either simple in shape like the so-called “Seabee” or “Hollow” shapes or more complex in shape such as the “Cob” or “Shed” ones. These units are usually placed in a single, uniform layer and their resistance depends fundamentally on gravity, friction and interlocking between the armor units, forming an assembly. As in the previous case, these armor units are also quite resistant at the start of damage due to the tight interlocking between armor units, but they have a very brittle structural performance because they tend to present a progressive failure to destruction if any of the armor units fail.
Focusing on the armor units with random placement, and more specifically on the mentioned cubic and parallelepiped blocks which, as stated earlier, are robust and easy to manufacture and are used along many coasts such as those of Spain, these units have serious drawbacks due to face-to-face fitting or self-arrangement. The reason is the parallelepiped design tends to pack armor units in the lower areas with small movements, forming armor layers with many planar faces coupled in the critical areas at sea level, decreasing the water friction on the slope of the mound and thus allowing a higher run-up and overtopping and even the flooding of the rear side of the breakwater if the wave is very large.